So I dont know how to explain it but i'm going to try my best to explain what happened at least 4 times today while farming for seals. It is possible to get 6 seals in this instance rather than 3. i have the screenshot to prove it. All I know is that if two people are taking care of the bookshelves, and one player leaves fellowship but not the instance before the 10th bookshelf activates, the second player can go to the last remaining bookshelf and activate it and get credit for the quest twice. i really dont know how else to describe it but here is a screenshot of proof you can get 6 seals.
ScreenShot00000 by LJCPhotos, on Flickr
as you can see, someone left fellowship at 9 shelves extinguished, and i got credit, went to do the 10th and got credit again. Just bringing this to light. dont hate on me please!